1. Field of the Invention
The present invention relates to a hexagonal-cell honeycomb catalyzer for purifying an exhaust gas from internal combustion engines mounted in for example vehicles and an exhaust gas purifying apparatus equipped with the hexagonal-cell honeycomb catalyzer.
2. Description of the Related Art
In recent years, systems for purifying an exhaust gas from internal combustion engines, such as on-vehicle engines, have been used for environmental measures. One of this kind of systems, a catalytic converter system is known, in which a support that carries catalytic materials including noble metal is arranged to an exhaust pipe. The support purifies HC (hydrocarbon), CO (carbon monoxide), NOx (nitrogen oxides), and other components in the exhaust gas by performing an oxidation reaction or oxidation-reduction reaction.
In general, this catalytic converter system is a honeycomb-shaped support (hereinafter referred to as “honeycomb support”) having a large number of cells. The honeycomb support, which is made up of ceramics, has cells into each of which a catalyzer is applied. The catalyzer has a catalyst layer containing catalytic materials.
It has been known that a large amount of HC is exhausted when an internal combustion engine starts, because of its lower temperatures. In view of purifying this HC, Japanese Patent Nos. 3489049 and 3311051 and Japanese Patent Laid-open Publication Nos. 11-179158 and 9-228828 disclose a structure in which HC absorption materials, such as zeolite, are added to the catalyst layer.
Of these publications, Japanese Patent Laid-open Publication No. 11-179158 proposes a catalyzer provided with a honeycomb support which has an HC absorption material layer essentially made up of zeolite and a catalytic component layer, laminated on the HC absorption material layer, containing catalytic components (materials) such as noble metal. In this catalyzer, the HC is absorbed by the zeolite in a period during which an internal combustion engine is just started, and its temperature is not high to cause the catalytic component to be activated sufficiently. The temperature of the exhaust gas will rise gradually after the engine start, whereby the catalytic component becomes activated to purify the HC desorpted from the zeolite.
However, the above catalyzer specification with the catalyst layer made up of the absorption and purification layers is still silent about how both the two layers should be best supported for efficiently purifying the HC in the exhaust gas during the engine start.
The honeycomb support, which supports the catalyst layer and others, has cells each of various shapes such as a square or a hexagon. Compared to the honeycomb support with the square cells, the honeycomb support with the hexagonal cells (hereinafter referred to as a “hexagonal-cell honeycomb support”) is advantageous in that the latter support provides a lower pressure loss, more improvement in purification by enlarging the support area of the catalyst layer, more even support of the catalyst layer, and others.
In such circumstances, such a catalyzer has been desired, which uses a hexagonal-cell honeycomb support as a base member, the support being provided with two layers, that is, a layer for absorbing HC and a layer for purifying HC using catalytic materials (hereinafter referred to as a “hexagonal-cell honeycomb catalyzer”), and which is able to efficiently purify HC in the exhaust gas when an internal combustion engine is started.